The decay of wood and other cellulosic materials by fungi, and the consumption of wood by termites, cause significant economic loss. Until recently, the most widely used wood preservative has been chromated copper arsenate (CCA). However, production of CCA for use in residential structures was prohibited as of January 2004 due to issues raised concerning the environmental impact and safety of arsenic and chromium used in CCA-treated lumber. As CCA replacements, arsenic-free and chromium-free wood preservatives are sought. Retention in treated wood of copper and other metal ions that are effective fungicides in treated wood is a challenge. Metal salts are generally water-soluble and rapidly leach from treated wood, which causes loss of the preservative function.
Tungstate and molybdate ions are toxic to termites, shipworms, and other wood-eating pests. Compounds containing these ions disrupt the symbiotic relationship between the wood-eating pest and the microbes in its gut which break down cellulose and fix nitrogen providing nutrients to the host. Nitrogenase, the microbial enzyme involved in nitrogen fixation, is a molybdenum-containing enzyme. It is known that tungstate can compete with molybdate to prevent microbial nitrogen fixation. Thus molybdate and tungstate toxicity to termites is thought to be related to nitrogenase, nitrogen fixation, and secondarily to cellulose degradation, related to microbial symbionts in the gut of the wood-eating pest.
Various molybdate and tungstate compounds have been used as termiticides. Sodium molybdate and sodium tungstate have been shown to be effective in termite killing baits (Brill et al., Naturwissenschaften v 74 p 494-495 (1987); JP 2001/097808 A). However, these compounds are too water soluble to provide a long lasting wood preservative. Treatment of wood surfaces with various inorganic compounds including copper molybdate in an aqueous mix has been described (Black and Mraz, Forest Service Research Paper, v 232 p 40 (1974); JP 2000/141316). The surface coating is not sufficient for wood protection.
U.S. Pat. No. 4,504,468 discloses treatment of cellulosic material for termite control with soluble metal salts of molybdate and tungstate. However, being soluble, these compounds leach from the treated material and therefore do not provide a lasting preservative.
JP-A 02/006,402 discloses a two-step process for retaining molybdate and tungstate in a cellulose based material. In the first step, wood is impregnated with a water soluble molybdic acid salt and/or tungstic acid salt, followed in a second step by addition of an inorganic acid or acid salt to reduce the solubility of the molybdate or tungstate ions. This two step process is very cumbersome and costly for commercial use.
Reduced leaching was obtained in using preservatives by replacing the arsenate of CCA with molybdate or tungstate (Cowan and Banerjee, Forest Products Journal v 55 p 66 (2005)). The toxicity of the chromium salts in these preservatives remains an issue in disposal of treated wood by incineration.
EP-B1 238,413 discloses a wood preservative containing a quaternary ammonium salt having wood preservative property, a water soluble inorganic and/or organic copper salt, ammonia and/or a water soluble amine capable of forming a complex with copper ions, and a molybdic acid compound and/or a nitrite. A quaternary ammonium salt is very costly for commercial use in wood preservatives since no method is provided to retard leaching loss of the very soluble quaternary ammonium salt.
U.S. Pat. No. 4,409,358 discloses a crop protection agent that is a copper amine salt of a polymer or copolymer of acrylic acid and/or methacrylic acid and optionally a lower alkyl ester of acrylic acid or methacrylic acid. U.S. Pat. No. 5,242,685 discloses a crop protection agent for controlling fungi or bacteria that is an aqueous solution of a polymer acid, containing acrylic acid or methacrylic acid and optionally acrylate or methacrylate, and at least 12% of copper, where the copper is dissolved by applying ammonia gas under pressure. The expense in making acrylic acid or methacrylic acid, and the requirement for 2 moles of monobasic (meth)acrylic acid groups per mole of Cu make this type of agent undesirable for commercial preparation.
U.S. Pat. No. 4,175,090 discloses a process for preparing a solution containing a cuprammonium complex of one or more C1 to C4 monocarboxylic acids. This type of complex would readily leach from treated wood and thus not provide a lasting preservative.
There remains a need for wood preservatives that are highly penetrating, effective, long lasting, and easily prepared for replacement of the CCA wood preservative.